Cellular and trunked radio communications systems are well known in the art and work to provide seamless communication as a subscriber unit moves about a geographic area covered by base sites within a predefined network. Most often, the subscriber and base units communicate using various types of linear modulation techniques that typically require a receiver with a substantially high dynamic range. Several types of problems are often associated with using such a receiver such as intermodulation (IM) distortion and blocking. This type of interference can be produced by high-power radio services using public service bands that can cause problems in establishing reliable communications between the base site and subscriber.
As best seen in prior art FIG. 1, a typical cellular network 100 operates with a plurality of target base sites 101-106 which are tied to a source base site 107. As a subscriber radio 109 travels through a site such as target base site 106, the site works to coordinate handoff to adjacent sites through the source base site 107. Problems regarding interference and handoff have been addressed in the prior art, and include such solutions as U.S. Pat. No. 6,266,530 entitled Multi-Cell Communications System and Method for Allocating a Communications Resource and U.S. Pat. No. 6,055,427 entitled Hard Handoff and Radio System which are both herein incorporated by reference.
Generally, there are two methods used in handing off subscriber units between cell sites. One method is the site-assisted method where the cell site measures the received signal strength indication (RSSI) of the subscriber's transmitted signal to determine when to handoff a subscriber to an adjacent site. A second handoff strategy is unassisted where a portable subscriber unit typically estimates carrier power on one cell site and moves to another site when the adjacent site carrier power measurement exceeds some threshold condition. There are many different algorithms that are used by either the cell site or the subscriber unit but each is similar in that the algorithm indicates to either the cell site or subscriber unit when the switch is going to occur.
One problem that may occur to the receiver in the subscriber unit is degraded reception due to intermodulation (IM) interference by strong in-band interference signals. This can greatly interfere with the subscriber's ability to receive reliable communications. Although various methods have been employed in order to reduce IM interference with the receiver, these methods often required the receiver to search and select alternative cell sites based on receiver performance for a particular channel. Alternatively, a cell system may depend on multiple transmission attempts where the same message is transmitted from a given cell repeatedly until reception is achieved. Both of these techniques effectively reduce system capacity by increasing redundant messaging or by increasing excess reuse of certain cell sites. Accordingly, there is a need to provide a method for mitigating IM interference without reducing system capabilities or capacity.